In a case where machining processing is performed over all the surfaces of a workpiece, a clamp device as described above (so called hole clamp device) is employed, since it is not possible to employ a clamp device of the type that presses from above on an end portion of the workpiece with a pressing implement. With such a clamp device, a seating surface is formed on a body member, and the workpiece to be the object of clamping is seated on the seating surface. When the workpiece is to be loaded, the workpiece is mounted on the seating surface and is supported thereupon, and a grip member and a clamp rod having a tapered shaft portion inserted into the grip member are inserted into a hole in the workpiece. And next, after increasing the diameter of a grip claw with the tapered shaft portion and engaging it against the inner circumferential surface of the hole by driving the clamp rod in a retraction direction and pulling it towards the seating surface, the workpiece is fixed against the seating surface by further pulling the grip member towards the seating surface. Clamp devices like the one described above are disclosed in Patent Documents #1 and #2.
Now, generally, often a seating sensor is provided to the clamp device, in order, when fixing a workpiece with the clamp device, to detect whether or not the workpiece has been fixed in the state of being correctly seated to the expected seating surface. This seating sensor comprises a pressurized air ejection hole that opens to the seating surface, an air supply passage that supplies pressurized air to the pressurized air ejection hole, a pressure switch that detects whether the air pressure within the air supply passage has risen to at least a predetermined pressure.    Patent Document #1: Japanese Patent No. 3,550,010 Publication    Patent Document #2: German Patent No. 4,020,981 Publication
Since the seating sensor only detects that the workpiece has seated against the seating surface, this does not limit the workpiece to being fixed with the predetermined clamping force, even though seating of the workpiece against the seating surface has been detected by the seating sensor. For example, in the case of the above described hole clamp device, when during clamping the clamp rod is driven in its retraction direction for clamping and the grip claw portion of the grip member is forced by the tapered shaft portion of the clamp rod to expand and to bite into the inner circumferential surface of the hole in the workpiece, sometimes it may happen that the grip claw portion slips relative to the hole in the workpiece, although the workpiece maintains its state of being seated against the seating surface. In particular, if the workpiece is a cast product, and if the hole therein is formed as a slightly tapering aperture, then slipping of the grip claw portion can easily occur.
If this type of slipping of the grip claw portion has occurred, then the workpiece comes to be clamped with a clamping force that is remarkably decreased, since the piston member of the fluid pressure cylinder shifts to retract as far as its limit retraction position. However it is not possible to detect the above type of poor clamping with the seating sensor, since even in this case the workpiece is held seated upon the seating surface. If machining of the workpiece is performed while it is held with this type of poor clamping, then there is the problem that the cutting tool may suffer damage or the workpiece may suffer damage, since the workpiece may be pushed away from its proper position by the cutting force from the cutting tool acting upon it.